On the basis of their morphology the cells of the periodontal ligament (PDL) appear to comprise a homogenous population. However, their response to various types of stimuli suggests that the population consists of fibroblasts and their precursors as well as the progenitors of osteoblasts, cementoblasts and possibly osteoclasts. Since it is not possible to distinguish these progenitor subpopulations morphologically little is known about their nature or their relationship to the functional fibroblasts. It is the major goal of the proposed project to characterize the nature of the response of PDL fibroblasts to specific stimuli wth the aim of understanding their functional and differentiative capacity. Two basic forms of stimulation are being applied to the periodontium. The first, orthodontic tooth movement, is being used to study the osteogenic response of PDL fibroblasts. By examining certain regions along the alveolar bone surface as well as areas in the central portion of the PDL it is possible to examine the nature of both the osteogenic reponse as well as a fibrogenic response. The latter is involved in repair of damage inflicted on the PDL by tooth movement. The second system being employed in these studies is one which involves a bone resorptive response. In this model parathyroid hormone administartion is being used to stimulate alveolar bone resorption. The nature of the response of PDL fibroblasts during the resorptive response is being assessed to determine if the osteoclasts generated arise from local progenitors or if they are derived from a blood born precursor.